brittle and brutal 21-month-long civil war. syrian regime shells landing inside turkey, prompting turkey's request to nato. that tension has died down. what has changed is the escalation in the conflict. rebels in the north pressing down hard on friday on a helicopter base, which they're trying to take to stop the regime's ability to project air power across the north. as the assad regime gets more desperate, many are concerned they'll reach into their arsenal, perhaps as the more deadly weapons inside, chemical weapons, which they deny having, scud-type missiles have already been used. purely to defend turkey, nato says. at the end of the day, as some say, they have skin in the game. perhaps were a shell to go astray, that may cause some ramifications for nato's patriot deployment there. all eyes watching this and certainly nato making the key point this is purely defensive. now certainly psychologically on what's happening in that border area. nick paton walsh, cnn, istanbul. >>> much more ahead this hour. >> here's what's co